Pre-coding is a kind of beam-forming that supports multi-layer transmission in MIMO radio systems. It has been shown that pre-coding increases the performance of wireless communication systems and has therefore been included in the standardization of 3GPP Long Term Evolution (LTE).
In pre-coding, each of a multiple of data streams are emitted from transmit antennas at a base station with independent and appropriate weighting per each antenna such that data throughput is maximized between the base station and user equipment. Pre-coding weights are calculated at the user equipment, which then informs the base station about which pre-coding weights are to be used. Usually, only a limited number of predefined pre-coding weights are used, called a codebook. The codebook is known by both the base station and the user equipment, so when the user equipment informs the base station about which set of pre-coding weights to use, the user equipment only needs to send a number corresponding to an index that the pre-coding weights have in the codebook. This number is usually called Pre-coding Matrix Indicator (PMI).
Studies have shown that a beam width of an antenna element pattern of a three-sector base station antenna should be about 65 degrees in order to optimize performance of a communication system. However, in case multiple antennas spaced one-half wavelength λ/2 apart are used at the base station, it is difficult to obtain antenna element patterns with beam widths as small as 65 degrees. Generally, the antenna element pattern will be around 90 degrees when the antenna separation is λ/2. In order to achieve antenna element pattern with beam width of the desired 65 degrees, the antenna separation could be increased, With antenna separation of e.g. about 0.8λ, the beam width of 65 degrees of the antenna element patterns could be obtained. However, when increasing the antenna separation above λ/2 another difficulty arises in that large gating lobes will occur when pre-coding beam are used that is steered out from broadside.
The above situations of different antenna separations and corresponding beam widths are illustrated in FIGS. 1 and 2 for the three sector base station. In particular, FIG. 1 illustrates an antenna configuration 1 of a three sector base station. That is, the base station is located in a cell of the communication system the coverage of which is divided into three sectors. The antenna configuration 1 comprises 4 active antenna elements 2, 3, 4, 5 having the antenna separation of 0.8λ. The antenna separation of 0.8λ and the beam width of the antenna element pattern are designed to be 65 degrees, the 65 degrees antenna element pattern being indicated at reference numeral 6. As mentioned above, with this antenna separation grating lobes 7 will occur when pre-coding beams 8 are steered away from broadside. This will increase the interference towards other cells and reduce the efficiency of the base station since power is transmitted in undesired directions.
FIG. 2 illustrates another antenna configuration 10 of the three sector base station. Here, the antenna configuration 10 again comprises 4 active antenna elements 12, 13, 14, 15 but now having the antenna separation of 0.5λ. This efficiently eliminates grating lobes when pre-coding. However, it is difficult to achieve an antenna design with a beam width, indicated at reference numeral 16, of less than 90 degrees for the antenna element pattern. In the typical three sector base station this results in high level of interference from one sector towards the other sectors of the cell.
The desire to obtain the beam width of 65 degrees is thus in conflict with the desire to use an antenna separation of about λ/2, both desires stemming from an effort to optimize the performance of the communication system. The above example illustrates the difficulties of simultaneously adapt different parameters, namely the beam width and the antenna spacing, to provide the best possible performance of the communication system.